The present invention relates to a semiconductor device utilizing a heterostructure and a method for fabricating the same.
In order to achieve higher speed operation and improved performance of semiconductor devices, an art to grow crystal of a semiconductor material whose band structure and lattice constants are different from those of the material of the substrate, an art to perform the so-called hetero-epitaxial growth, is attracting considerable attention from various fields. Earlier, intensive studies were made on the art with compound semiconductors of GaAs or the like which have high mobility, and such high-speed elements as modulation doped field effect transistor (MODFET), high electron mobility transistor (HEMT), and hetero-junction bipolar transistor (HBT) were experimentally produced and announced. Quite recently, an art to epitaxially grow, on an Si substrate, such a semiconductor material as Si.sub.1-x Ge.sub.x (0&lt;x.ltoreq.1), SiC, and GaAs, which is different from Si in band structure and lattice constant, or fabrication of a device of heterostructure using such an art has been becoming the object of intensive studies in various fields.
When growing crystal of material different from the substrate in lattice constant, there has been a big problem. As an example, a case where a crystal of an Si.sub.1-x Ge.sub.x alloy different from Si in band structure and lattice constant is grown on an Si substrate will be considered. Since Si and Ge have different lattice constants, there is produced a lattice mismatch between Si and Si.sub.1-x Ge.sub.x alloy and its value varies from 0% to 4% depending on the alloy ratio (x). Since the Si.sub.1-x Ge.sub.x alloy film which is crystallized on the Si substrate grows in conformity with the lattice constants of the Si substrate as the base material, it receives compressive stress. This stress gives such merits to the device, in terms of electric property, that increases the band discontinuity between the Si substrate and the Si.sub.1-x Ge.sub.x alloy and that decreases the effective mass of holes. However, the heterostructure formed under the described conditions comes into an unstable state. For example, as the Si.sub.1-x Ge.sub.x alloy film increases in thickness, dislocations are produced at the interface between the film and Si to relax the lattice mismatch and the characteristics of the device are deteriorated on account of the produced dislocations. The film thickness at which the dislocation is produced is called the critical thickness. Further, since the strain in the grown film is relaxed, the advantageous characteristics of the heterostructure cannot be sufficiently utilized. Even under the condition of the film thickness below the critical thickness, dislocations are produced by a high-temperature process such as heat treatment given to the film, and thereby, the strain in the grown film is relaxed and the film tends to come into a stable state. Therefore, in order to fabricate an element fully exhibiting advantages of the heterostructure, it is essential that a technology to grow crystal for forming a dislocation-free, thermally stabilized, and greatly strained film be established.
A typical example of Si/Si.sub.1-x Ge.sub.x /Si HBT making use of a heterostructure is disclosed in IEDM. Tech. Dig., pp. 874-876 (1987). A sectional view of the HBT is shown in FIG. 2. In this case, the base layer 21 of Si.sub.0.88 Ge.sub.0.12 with a smaller band gap than Si and the emitter layer 22 of n-type Si were both formed by molecular beam epitaxy, an SiO.sub.2 film 26 as a protecting film is formed by CVD (chemical vapor deposition), and the emitter electrode 23, collector electrode 24, and base electrode 25 were formed by vacuum evaporation. Since the base layer 21 of Si.sub.0.88 Ge.sub.0.12 is grown with strain on the Si substrate 20, there is produced a band discontinuity of the valence band of 0.1 eV at the junction between the same and the emitter layer 22 of Si. Because of the presence of this band discontinuity of the valence band, injection of holes from the base layer 21 to the emitter layer 22 is suppressed and, hence, high emitter efficiency can be obtained.
In IEEE Dev. Lett. DL-7, pp. 308-310 (1986), there is disclosed an MODFET with a strained SiGe alloy film grown on an Si substrate as the channel layer and making use of two-dimensional hole gas produced in the band discontinuity. A sectional view of the element is shown in FIG. 3. In this element, an Si.sub.0.8 Ge.sub.0.2 channel layer 31 and a p-type emitter layer 32 were epitaxially grown in succession on an Si substrate, an SiO.sub.2 film 37 was formed as a protecting layer by CVD, and the source electrode 34, gate electrode 35, and drain electrode 36 were formed by vacuum evaporation. Denoted by 33 is a high density p-type Si layer. It is confirmed that this element functions as a MODFET by virtue of two-dimensional hole gas generated in a band discontinuity at a hetero-interface between the Si.sub.0.8 Ge.sub.0.2 channel layer 31 and the p-type emitter layer 32. Incidentally, the film thickness of the Si.sub.0.8 Ge.sub.0.2 channel layer 31 is set to be below 25 nm which is the critical thickness of the strain-grown film with respect to the Si substrate 30.
In the above described prior art MODFET, since the thickness of the channel layer 31 of Si.sub.0.8 Ge.sub.0.2 has been set to be below the critical thickness, the channel layer has not been given a sufficient thickness for functioning as the channel layer, and therefore, there has been a problem for the element to be hindered from making high-speed operation by the scattering of carriers.
Further, in the above prior art HBT and MODFET, it has been unable to increase the alloy ratio x in order to suppress occurrence of dislocations in the hetero-grown film and, accordingly, the value of the band discontinuity at the hetero-interface between the base layer 21 formed of Si.sub.0.88 Ge.sub.0.12 and the Si substrate 20 or the hetero-interface between the channel layer 31 formed of Si.sub.0.8 Ge.sub.0.2 and the Si substrate 30 has been as low as from 0.1 to 0.15 eV. Because of the low value of the band discontinuity, it has not been possible to accumulate the two-dimensional carrier gas in a high density and to fully obtain meritorious effects of the heterostructure.
In order to increase the value of the band discontinuity and to accumulate two-dimensional carrier gas in high density, there is a method for example to increase the Ge content to 30% or higher so that the the band gap between the layer and the Si substrate is increased. In such case, however, the lattice mismatch exceeds 1% and, hence, the Si.sub.1-x Ge.sub.x alloy film exceeds the critical film thickness with respect to the Si substrate so that dislocations are introduced at the interface and the strain is relaxed. On the other hand, when the film thickness is below the critical thickness, there arises a problem that a sufficient thickness as the base layer or the channel layer cannot be obtained.
Further, in either case, there has been a problem that dislocations are introduced into the grown film in the process of device fabrication after the growth of the film, such as heat treatment following the ion implantation.
In Japanese Patent Publication No. 63-503104 (International Publication No. WO 87/06392), a case is disclosed, in which an epitaxial layer having different lattice constants from those of the substrate was formed on the substrate having a fine pattern, and a semiconductor device was fabricated in the epitaxial layer. In this case, the epitaxial layer was formed on pattern plateaus, the maximum size of the plateau being smaller than 200 angstrom units. It is considered very difficult to form one or more elements on such a fine pattern plateau.